In recent years, ultrathin IC chips which are built in ultrathin IC cards and the like represented by a smart card have been required. Such ultrathin IC chips are manufactured by dividing an ultrathin wafer of not more than 100 μm into individual chips.
Against this background, in conventional methods for dividing a plate-like member for semiconductor devices, electronic parts and the like, as shown in the flowchart of FIG. 7, a protective tape sticking step (Step S101) is first performed. In this protective tape sticking step, in order to protect a wafer surface on which may semiconductor devices, electronic parts and the like are formed, a protective tape having an adhesive on its one side is first stuck to a wafer surface. Subsequently, a back surface grinding step (Step S103) is performed. In this back surface grinding step, the wafer is ground from its back surface and worked to a prescribed thickness.
After the back surface grinding step, a frame mounting step is performed which involves mounting the wafer on a dicing frame by use of a dicing tape having an adhesive on its one surface and the wafer and the dicing tape become integrated (Step S105). Subsequently, a protective tape removing step is performed which involves adsorbing the wafer in this condition on the dicing tape side and removing the protective tape stuck to the surface (Step S107).
The wafer from which the protective tape has been removed, along with the frame, is transferred to a dicing saw and cut into individual chips by a diamond blade which rotates at a high speed (Step S109). Subsequently, in the expanding step the dicing tape is radially elongated and the intervals between individual chips are widened (Step S111), and in the chip mounting step the chips are mounted on package substrates such as lead frames (Step S113).
However, in such conventional methods for dividing a plate-like member, it was necessary to use a protective tape for preventing the pollution of a wafer surface during the grinding of the back surface of the wafer and a dicing tape for holding chips after dicing and this lead to an increase in the cost of consumables.
Furthermore, in the case of an ultrathin wafer having a thickness of not more than 100 μm, under conventional methods by which a wafer is cut by use of a dicing saw, chipping and breakage are formed during cutting, thereby posing the problem that good chips become defective products.
As means to solve the problem that chippings and breakage occur in a wafer during cutting, there have been proposed techniques related to a laser processing method which involves, in place of conventional cutting by a dicing saw, causing laser beams to become incident, with a condensing point aligned in the interior of a wafer, forming a modified region in the interior of the wafer by multiphoton absorption, and then dividing the wafer into individual chips (refer to, for example, the Japanese Patent Laid-Open No. 2002-192367, the Japanese Patent Laid-Open No. 2002-192368, the Japanese Patent Laid-Open No. 2002-192369, the Japanese Patent Laid-Open No. 2002-192370, the Japanese Patent Laid-Open No. 2002-192371, and the Japanese Patent Laid-Open No.2002-205180).
However, in the techniques proposed in the above-cited unexamined patent publications, a dicing device by a dividing technique by use of laser beams is proposed in place of a conventional dicing device by a dicing saw, and although the problem that chippings and breakage occur in a wafer during cutting is solved, the problem that in the expanding step, portions which are not divided are formed and end-face shapes of divided chips become poor.
FIGS. 8 and 9 are each a conceptual figure to explain this phenomenon. In FIG. 8, a dicing tape S is stuck to the back surface of a wafer W and the peripheral edge portion of the dicing tape S is fixed to a frame F. A modified region K in rectangular arrangement is formed in the wafer W by laser beams. Subsequently, in the expanding step, the dicing tape S is elongated, with the result that the wafer W is divided into multiple chips T, with the modified region serving as starting points.
The elongating of the dicing tape S in the expanding step is performed, for example, by pushing up a cylindrical ring member from below in an annular portion between the frame F of the dicing tape S and the wafer W.
FIG. 9 is a schematic diagram to explain the division of a wafer W in an expanding step. FIG. 9(a) is a plan view and FIG. 9(b) is a sectional view. As shown in FIG. 9(b), a modified region K formed by laser beams is present in the interior of the wafer W. As shown in FIG. 9(a), cutting is performed satisfactorily when a uniform tension is applied to the wafer W.
In conventional methods and apparatus for dividing a plate-like member, however, it is often that in the expanding step, the elongation of the dicing tape S does not become uniform all over the surface of the wafer W. For example, the dicing tape S in a portion where cutting has already been performed is locally elongated, and it becomes impossible to apply a tension to the dicing tape S in uncut portions. As a result, it is often that uncut portions are formed and that the end-face shapes of divided chips do not become linear and become poor.
The present invention has been made in view of such circumstances and has as its object the provision of a method and an apparatus for dividing a plate-like member which can positively manufacture an ultrathin chip with a good end-face shape in which uncut portions, chippings and breakage do not occur when, after working a wafer to a prescribed thickness by grinding the back surface of the wafer, working for forming a modified region by laser beams is performed and the wafer is divided into individual chips.
Also, the present invention has as its another object the provision of a method and an apparatus for dividing a plate-like member which can miniaturize the apparatus for dividing a plate-like member and can perform dividing work of a plate-like member in a short time.